In case-hardening, steels of low carbon content are annealed in carbon-releasing agents at temperatures of between 800.degree. and 950.degree. C. The surface is enriched with carbon and becomes hard on quenching. The carbon-releasing agents used are in most cases endothermic gases which contain about 20% of CO, 40% of H.sub.2 and 40% of N.sub.2. During the carburization of these steels with endothermic gas, an oxidation of the base alloy elements occurs in the surface zone of the steels, so that these are no longer present during the later formation of the microstructure. In the surface zone of the steels, an undesired microstructure forms in this case, which has unfavorable properties and requires mechanical removal or sandblasting of this surface zone in order to obtain the required properties of the steels (workpieces).
Investigations have shown that this surface oxidation is essentially caused by the oxygen potential of the endothermic gases used, even though these gases have a strongly reducing action and no "free oxygen" is present at the particular carburization temperature. The oxygen activity is determined by the contents of CO, CO.sub.2 and H.sub.2 O and by the non-oxygen-containing components (H.sub.2 and CH.sub.4). The dominating carburization part reaction in such CO-containing gas atmospheres is the carbon monoxide decomposition on the workpiece surface: EQU CO.sub.gas =[C].sub.dissolved +[O].sub.adsorbed
The released carbon, and also the adsorbed oxygen produced in the reaction, are dissolved by the alloy and diffuse into the steel. The quantity of dissolved oxygen is determined by the oxygen activity of the gas phase and by the duration of the treatment time and it is very much smaller than the quantity of carbon being dissolved. The oxygen solubility in pure iron is approximately 0.0003% by weight of oxygen (3 ppm of oxygen) at 950.degree. C. and a C level of 1% by weight of carbon when an endothermic gas of methane is used.
If the oxygen partial pressure for the formation of a metal oxide is exceeded, oxidation of the particular metal takes place. EQU Me+H.sub.2 O=MeO+H.sub.2 EQU Me+CO.sub.2 =MeO+CO
The oxygen potential of the carburization media used is as a rule so low that no oxidation of the iron takes place. Alloy elements present in the steels, however, have a high oxygen affinity, so that small quantities of dissolved oxygen in the alloy lead to the so-called internal oxidation.
Conventional alloy elements are: Cr, Mn, Si, Ti, V and others which are present in low concentrations. Surface oxidation or also internal oxidation is understood as precipitations of oxides of the abovementioned metals within a metal grain or along the grain boundaries, which precipitations are formed by the dissolved oxygen diffusing in and are then dispersely distributed in the matrix.
The kinetics of the oxygen uptake obey a diffusion-controlled time law, and the depth of penetration thus increases parabolically with the duration of carburization. The depth of penetration of the oxygen and the thus resulting depth of surface oxidation can be calculated by the following equation: ##EQU1## X.sub.t depth of penetration of the oxygen D.sub.o diffusion coefficient of the oxygen in the alloy
C.sub.o oxygen concentration from the alloy surface PA0 C.sub.ME concentration of the base metal in the alloy (for example silicon) PA0 .nu. stoichiometric factor PA0 1st stage main carburization phase PA0 2nd stage diffusion phase